In recent years, the integration and precision of semiconductor devices have become increasingly higher, and the diameter of a wafer to be used has become steadily larger, and demand for the wafer having a large diameter of particularly 150 mm, 200 mm, and 300 mm or more has been increased.
Incidentally, there has been a problem that dislocation is easy to be generated during growing a large diameter crystal by the CZ method, since the growth of the crystal having a diameter of 150 mm or more was started. Particularly, in case of manufacturing a large diameter crystal doped with nitrogen or a large diameter crystal having a low resistivity, the generation of dislocation is significantly increased.
After the steps of melting a crystalline raw material charged in a crucible and of decreasing the temperature of a melt so as to be an appropriate status of the melt for the start of crystal manufacture are performed, the crystal manufacture can be started. Conventionally, it is attempted to start the crystal manufacture as soon as possible by shortening the time required for the above-described steps in order to shorten the time required for the whole manufacture including the melting, decrease in temperature, and crystal manufacture.
With a larger diameter of a single crystal, however, productivity for conditions of growing the single crystal having a low resistivity may become worse by the above-described method. Recently, there is an art for attempting to improve the productivity by providing with a step for maturing the melt after melting the crystalline raw material to prevent unmelted remains of, for example, a crystalline raw material or dopant (See Patent Literatures 1 and 2).
When the crystal is pulled with DF (Dislocation Free) without the generation of dislocation, loss of time and product is small. The dislocation is practically generated with some frequency. Once the dislocation is generated, the dislocation is introduced into a dislocation free portion at approximately a length corresponding to the diameter thereof. The portion of the generated dislocation cannot be a product naturally. In the event that the length of the crystal portion that can be a product is short, the crystal is melted again. On the other hand, in the event that the length of the crystal portion that can be a product is equal to or more than a predetermined length, since product parts can be secured to a certain extent, it may be taken out, as it is, without remelting. In the former case, even if the crystal is pulled with dislocation free to the end after remelting, there occurs time loss of crystal manufacture time before the remelting plus remelting time plus re-decreasing temperature time, and productivity consequently becomes worse. In the latter case, the portion of the generated dislocation cannot be a product, and both of yield and productivity consequently become worse simply.
A value obtained by converting the frequency of conducting the remelting of the crystal into an average per one crystal is referred to as a remelting ratio. A value obtained by converting the number of the crystal pulled with DF to the end among all pulled crystals into a value per the whole number is defined as a DF ratio. Conventionally, in case of a small diameter (that is, the crucible also has a small diameter), the remelting ratio is low. On the other hand, in case of a large diameter (that is, the crucible also has a large diameter), the remelting ratio is high. The DF ratio in case of a large diameter also becomes worse in comparison with that in the case of a small diameter.